Today the majority of pregnant women in the United States are either overweight or obese at conception with their offspring having greater adiposity at birth, a 2-fold greater risk of later obesity, and neonatal insulin resistance. Breastfeeding ha many clear benefits that may include protection against obesity and its sequelae, and exclusive breast milk feeding is recommended as the ideal infant feeding strategy for the first six months. It was long thought that breast milk composition was fairly uniform among women, having been optimized through evolutionary time to provide adequate sole nutrition for the growing infant regardless of the environmental circumstances. However, recent evidence shows that breast milk is a highly complex fluid with significant inter-individual variation in hormonal and cytokine concentrations. Pervasive maternal obesity is an evolutionarily novel condition for the human species but little effort has yet been made to systematically examine how this novel condition is associated with breast milk adipose-tissue derived hormone and cytokine (adipocytokine) variation, or whether that variation relates to infant metabolic status. Our objective is to comprehensively assess the lactational programming hypothesis, that is, whether or not recently documented variation in breast-milk composition is related to both maternal adiposity and to infant metabolic status. The central hypothesis is that a graded, dose-response relationship between maternal adiposity and adipocytokine concentrations in breast milk exists and that milk adipocytokine concentrations are associated with altered body composition in their exclusively breast-fed offspring. Guided by compelling preliminary data, and with consideration of potential confounders, this hypothesis will be tested by pursuing three Specific Aims: 1) Identify windows of exposure during the perinatal period (pre-pregnancy, pregnancy, post-partum) when elevated maternal weight and weight gain are associated with milk adipocytokine concentrations; 2) Test novel relationships between breast-milk adipocytokine concentrations and infant body composition and insulin secretion; and 3) Further characterize the relationship of maternal adiposity to milk variation and infant outcomes, to augment Aims 1 and 2 using state-of-the-science maternal and infant body composition methods, comprehensive breast-milk sampling techniques, and metabolomic analysis. The proposed research is significant because it tackles an understudied, but potentially important pathway explaining the vicious cycle of maternal-child obesity transmission prevalent today. The results of the study will be used to design interventions to reduce maternal weight during pregnancy and lactation and to augment lactation education materials to focus on the needs of obese breast-feeding women. The ultimate goal is to support women and infants during this crucial period of developmental plasticity, for the prevention of obesity, diabetes and cardiovascular disease.